Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is sizing a common vent for two lavatories and one bathtub, all connected to the same branch. The lavatories each have a DFU of 1, and the bathtub has a DFU of 2. What is the minimum diameter for the common vent pipe if it serves these fixtures?
- A1.5 inches (38 mm)
- B2 inches (50 mm)
- C1.25 inches (32 mm)
- D3 inches (75 mm)
Show answer & explanationAnswer & explanation
Correct answer: A. 1.5 inches (38 mm)
Total DFU = (2 lavatories * 1 DFU/lav) + (1 bathtub * 2 DFU/bathtub) = 2 + 2 = 4 DFU. According to most plumbing codes, a common vent serving up to 8 DFU (depending on length) can be 1.5 inches (38 mm) in diameter. The minimum vent pipe size is generally 1.25 inches (32 mm), but this total DFU would require at least 1.5 inches.
Why the other options are wrong
- B. 2 inches would be oversized for this DFU load, though it would function adequately.
- C. 1.25 inches is the absolute minimum pipe size for a vent, but usually only for a single, low-DFU fixture.
- D. 3 inches is significantly oversized for this application and unnecessary.
Common Vent Sizing
Determining the appropriate diameter of a vent pipe that serves multiple fixtures, based on their total drainage fixture units (DFU).
- Based on total DFU served.
- Also considers developed length of the vent.
- Ensures adequate air circulation for all connected fixtures.
Memory trick: DFU, length, and pipe size, keep the air flowing, plumbing's prize!